Pipelines in the oil and gas industry are typically formed from multiple lengths of steel pipeline sections that are welded together end-to-end as they are being laid. To prevent corrosion of the pipeline sections and to reduce heat loss of fluids transported by pipelines, the pipeline sections are coated with one or more protective or insulative layers. The pipeline sections are usually coated at a factory remote from the location in which they are to be laid. This is often referred to as factory-applied coating and it is generally more cost effective than coating pipeline sections on site where they are laid. At the factory, the coating is applied to the outside of the pipeline sections whereupon a short length is left uncoated at either end of the pipeline section. The uncoated ends are necessary to enable the pipeline sections to be welded together to form the pipeline in the field.
Polypropylene coating has good protective and insulative properties and it is commonly used to coat pipelines transporting fluids at up to 140 degrees centigrade. Polypropylene is widely used for factory-applied coating for pipeline sections used to form pipelines. The pipe coating can take several different forms depending on the particular application and will normally consist of more than one layer. A conventional pipe coating will typically comprise a first thin layer of a primer, such as an epoxy-based material, that is applied in either liquid or powdered form to the outer surface of the steel pipeline section. To ensure a good bond between the pipeline section and the primer, the pipeline section is typically blast cleaned and etched with an appropriate anchor pattern. A second layer of polypropylene chemically modified to act as an adhesive will then usually be applied over the primer during the curing time (i.e. time taken to harden or set) of the primer. While curing of the primer is ongoing, and so as to allow the all the layers to bond, a third layer is applied. Typically, the third layer is polypropylene and a common process for coating pipelines with polypropylene is the Injection Molded Polypropylene (IMPP) technique. An IMPP coating is typically applied while the steel pipeline section is heated by induction heating, for instance. All but the ends of the pipeline section is enclosed by a heavy duty steel mold that defines a cavity around the uncoated pipeline section, which is subsequently filled with molten polypropylene from an IMPP injection molding machine in the factory. Control of the heating, so the factory-applied coating is sufficiently heated to allow fusion to occur when the molten polypropylene is introduced into the mold, requires skill. The mold must be of heavy duty construction, often incorporating hydraulic opening and closing mechanisms in order to withstand high molding pressures. The IMPP injection molding machine which dispenses polypropylene into the mold is normally closely coupled to the mold. Once the polypropylene has cooled and solidified, the mold is removed to leave the factory-applied coating in place on the pipeline section.
Before the pipeline can be laid the welded ends, known as field joints, must be coated in the region of the joint to prevent corrosion of the pipeline. The coating in these regions is referred to as the field joint coating. Two common processes for coating field joints of pipelines formed from polypropylene coated pipeline sections are the IMPP technique and an Injection Molded Polyurethane (IMPU) technique. Patent publication No. WO2009/027686 discloses a filed joint coating technique which permits fusion to occur between the pipe and the field joint coatings.